This code was using get_user_pages*(), in a "Case 2" scenario
(DMA/RDMA), using the categorization from [1]. That means that it's
time to convert the get_user_pages*() + put_page() calls to
pin_user_pages*() + unpin_user_pages() calls.

Factor out a new, small release_registered_pages() function, in
order to consolidate the logic for discerning between
TEE_SHM_USER_MAPPED and TEE_SHM_KERNEL_MAPPED pages. This also
absorbs the kfree() call that is also required there.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: tee-dev@lists.linaro.org
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>